Han Jae In was an exchange student from Beijing became a trainee at a Beijing company. The company decided to transfer her to YNB Ent to improve her training. Eventually, she completed her training within 3 months. Unfortunately, she couldn't go back to Beijing due to some financial problems at the company. YNB offered her to debut as a solo but she refused. Not wanting to waste her talent, YNB made her as a back-up singer for their first boyband group - KNK. Tinkerbells became aware of her voice in nearly all of their song. They began to question YNB and KNK of the mysterious female voice.